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=, ?= - Up Cast and Down Cast

Other versions: 7.31 | 7.40 | 7.54

Syntax


destination_ref =|?= source_ref.

Effect

Assignment between two reference variables. The reference in source_ref is assigned destination_ref. If the assignment is successful, destination_ref points to the same object as source_ref (reference semantics). The assignment of reference variables is a special form of assignments of data objects; assignments between reference variables require two assignment operators, used in accordance with the assignment rules for reference variables:

  • In assignments between reference variables, the assignment operator = can only be used for up casts in which the static type of source_ref is more specific than or the same as the static type of destination_ref.
  • The special casting operator ?= can only be used for assignments between reference variables. If the static type of source_ref is more general than the static type of destination_ref, ?= must be used to produce a down cast. If this is statically identifiable, it is checked by the syntax check; if not, it is checked at runtime. The actual down cast (that is, the check to see whether assignments are possible in accordance with the assignment rules for reference variables) never takes place until runtime. If, then, the static type of destination_ref is not more general or is the same as the dynamic type of source_ref, a handleable exception is raised and the target variable keeps its original value.

The same applies to the right side and left side as when assigning data objects, with the following restrictions:


Notes

  • The casting operator ?= can always be specified, even for up casts. This is, however, not usually necessary.
  • If it is possible to identify (statically) that assignments are not possible, neither = nor ?= can be used. This is the case, for example, when the static types of source variables and target variables come from different paths of the inheritance tree.
  • The null reference of an initial reference variable can be assigned to every target variable in a down cast that can be specified here.
  • Alongside ?=, the casting operator CAST also enables down casts to operand positions, which saves on helper variables.
  • The casting operator ?= cannot be used in multiple assignments.
  • One obsolete form of down cast is the statement MOVE with the addition ?TO.


Example

The first two assignments in the following source code section are up casts:

  • The instance operator NEW creates a result with the static and dynamic type c2, which can be assigned to the more general reference variable oref1.
  • Any reference variable can be assigned to the reference variable oref with the most general static type object.

The next two assignments are down casts:

  • It is not possible to test whether the general reference variable oref points to an object that can also point to oref2 until runtime. This is the case in the example.
  • The down cast of oref2 to oref3, however, fails at runtime and raises the caught exception. 

CLASS c1 DEFINITION INHERITING FROM object. 
ENDCLASS. 

CLASS c2 DEFINITION INHERITING FROM c1. 
ENDCLASS. 

CLASS c3 DEFINITION INHERITING FROM c2. 
ENDCLASS. 

DATA oref TYPE REF TO object. 
DATA oref1 TYPE REF TO c1. 
DATA oref2 TYPE REF TO c2. 
DATA oref3 TYPE REF TO c3. 

oref1 = NEW c2( ). 

oref = oref1. 

TRY. 
    oref2 ?= oref. 
  CATCH cx_sy_move_cast_error. 
    ... 
ENDTRY. 

TRY. 
    oref3 ?= oref2. 
  CATCH cx_sy_move_cast_error. 
    ... 
ENDTRY.

Exceptions


Catchable Exceptions

CX_SY_MOVE_CAST_ERROR

  • Cause: Type conflict in down cast
    Runtime Error: MOVE_CAST_ERROR
  • Cause: Source variable or target variable is not a reference variable
    Runtime Error: MOVE_CAST_REF_ONLY
  • Cause: Dynamic type conflict in assignment of references
    Runtime Error: MOVE_CAST_ERROR_DYN